Removal of ammonium nitrogen from wastewater is highly desirable because ammonium nitrogen exerts an oxygen demand in the receiving water, inhibits the disinfection of a wastewater, is toxic to aquatic life and is a nutrient contributing to eutrophication of natural waters. Many wastewater treatment facilities are now required to remove ammonium nitrogen from their effluents in the interest of environmental protection. Ammonium introgen removal may be accomplished by biological nitrification. Nitrification is the oxidation of ammonium nitrogen to nitrate nitrogen by nitrifying autotrophic bacteria. Complete nitrogen removal may be achieved by subsequent denitrification which is the reduction of nitrate nitrogen to molecular nitrogen by facultative heterotrophic bacteria under anoxic conditions.
The activated sludge process in its most common adaptations stabilizes wastewater biologically under aerobic conditions. Carbonaceous material in the wastewater is used as substrate for growth by microorganisms. The biological solids formed during the aeration of the sewage are separated in a sedimentation tank and recycled back to the aeration tank to continue the process. Excess biological solids are removed from the system as required. Some variations of the activated sludge process commonly in use are conventional aeration, step feed aeration, complete mix aeration, extended aeration, high rate aeration, contact stabilization and the pure oxygen system. Most modifications of the activated sludge system were developed primarily for the removal of suspended material and the oxidation of carbonaceous material by heterotrophic bacteria.
It is possible to achieve nitrification in almost any activated sludge system provided that conditions are adequate to maintain a population of the nitrifying autotrophic bacteria. These conditions include sufficient dissolved oxygen, carbonate alkalinity in excess to maintain appropriate pH and an adequate solids retention time. Nitrifying bacteria have a relatively low growth rate which generally prevents significant nitrifying populations from occuring in activated sludge systems with short solids retention times. Since the low growth rate of the nitrifying bacteria is further decreased with decreased temperature, the minimum solids retention time required to maintain a sufficient nitrifier population will increase as temperature decreases.
Wastewater treatment plants generally operate under variable conditions of flow, wastewater strength and temperature. Because of the dynamic conditions a treatment plant may be subjected to, a nitrifying activated sludge plant is usually designed to be able to maintain a solids retention time much greater than the minimum that would be required for steady-state operation under the most severe conditions encountered. Long solids retention times permit the buildup of a nitrifying population to a level which approaches the maximum attainable for the amount of ammonium nitrogen substrate available.
As a consequence of the ability to maintain long solids retention times, nitrifying activated sludge systems are usually characterized by low carbonaceous loadings and long hydraulic retention times. To maintain the long solids retention times and long hydraulic retention times nitrifying activated sludge systems often are much larger in size than non-nitrifying systems. The design and maintenance of nitrifying activated sludge systems under dynamic conditions is discussed in detail in the Process Design Manual for Nitrogen Control, U.S. Environmental Protection Agency, U.S. EPA Technology Transfer, Washington, D.C., 1975, and Poduska, R. A. and Andrews, J. F., "Dynamics of Nitrification in the Activated Sludge Process," Journal Water Pollution Control Federation, Volume 47, pages 2599-2619, 1975.
Granted adequate dissolved oxygen and non-inhibiting pH levels, nearly complete ammonium nitrogen removal can be maintained in nitrifying activated sludge systems except under stressful conditions including the following:
1. An increased wastewater flow through the system resulting in an insufficient hydraulic retention time for complete ammonium nitrogen oxidation.
2. An increased ammonium nitrogen loading to the system which may be too high for complete ammonium nitrogen oxidation.
3. Decreased wastewater temperatures which cause loss of nitrification efficiency at the solids retention time maintained in the system.
4. An increased carbonaceous loading to the system which results in a decreased solids retention time and decreased nitrification.
It is the object of this invention to improve nitrification in activated sludge systems thereby increasing nitrification reliability and reducing effluent ammonium nitrogen concentrations, particularly during periods of stressed operation.